Printed resistors and inks



a the resistance layer after resin'curing.

7 2,795,680 7 PRINTED RESISTORS AND INKS David B. Peck, Williamstown, Mass., assignorfto Sprague Electric Company, North Adams, Mass, a corporation of Massachusetts i No Drawing. Application May 16, 1952, Serial No. 288,305 I '4Claims. Cl. 201-63 This invention relates to improved electrical devices 'of finely divided conducting particles and from about such as resistors and specifically refers to improved re- I sistors of the printed type.

There are numerous forms and types of electrical resistors such as wire wound, carbon composition, carbon film, metal film, etc. One type which has come into some use is the so-called printed resistor which is pro duced by laying down through a silk or steel screen a suspension of carbon black or graphite in a solution of resinous binder, followed by the removal of solvent and curing of the resin. The base materials used for such resistors include Bakelite, ceramic and glass.

While such printed resistors are essentially an inexpensive type and theoretically capable of close manufacturing control, it is found in practical. operation that the resistors offer numerousproblems discouraging their use.

One typical and common problem is the stability of the printed resistor per se or resistor in a complex printed circuit. The usual processing steps which follow laying down of the resistor are curing of the resin, screening a protective resin coat over the resistor, curing this coat, soldering. of lead wires to the usually silvered ;contact areas, degreasing the rosin from the. soldered joints, applying an outer protective coat resin, curing this latter resin and finally vacuum wax impregnating the protective "'resinc'oat. The various heating operations involvedhere,

as well as the degreasing solvent and protective coat solvent,aifect the resistance value and characteristics of the printed resistor remarkably. Usually the resistance 'value is greatly increased by the various processing operations;

unfortunat ly, ,however, this increase is non-uniform, making it ditficult, if not impossible, to maintain the appropriate resistance value tolerances. "In some instances,' 7

actual control of the resistance value of the screened resistor is in itself difficult, due to viscosity problernsysuspension uniformityproblems and variations in the extent of resin polymerization during and following this screening operation. In such cases, it iscommonforthe operator to adjust the resistance value by scraping'off' some of Another disadvantage fof .present pri'ted resistors resides in the negative temperature coefiicient of resistance which is obtained. As a result, overloading of the resistor to even'aminor'degree'willresultin a drop in resistance value causing'a further overload and higher temperature, etc., until the resistor burns out. A positive temperature coeff cient of resistance wouldovercome this serious disadvantage.

, It is an object of the p'resent invention to overcome the foregoing andrelated disadvantages. A further object is to produce new and useful printed resistors. A

still 'further object is to produce printed resistors having-..

ent'invention wherein there is produced'a printed resistor 2,795,680 Patented June 11, 1957 ICC comprising an insulating base with terminal points upon which and against which, respectively, is deposited an adherent resistance layer consisting of a resinous binder selected from the class containing cross-linked epoxy resins and finely divided particles of conducting material.

' a more restricted sense, the invention is concerned with a printed resistor comprising a ceramic base, upon which is deposited an adherent layer of resistance material consisting of from about 50% to about 95% of a cross-linked epoxy resin, from about,50% to about 5% 25% to about 0% of finely divided non-conducting particles, terminal contact areas being provided at the ends of said layer. 7

The invention is also concerned with the preparation of resistor ink, consisting of a solution of a predominant amount of an epoxy type resin dissolved in a high boiling solvent, a significant amount of conducting particles suspended in said solution and minor amounts of a crosslinking agent, active only at temperatures exceeding C. In accordance with one of the preferred embodiments of the invention, the cross-linking agent is in the form of finely divided particles suspended in said solution.

It has been found possible to produce printed resistors with excellent electrical characteristics by a simpleand efiicient manufacturing process utilizing specifictypes of resins which are capable of reacting to substantially in- :soluble. and thermo-setting states under selected thermal processing conditions.

In the prior art, resins whichhave been suggested for usein printed resistors and resistor inks include the phenol-formaldehyde resins, the urea-formaldehyde resins, .the melamine-formaldehyde resins, the linseed soya and castor oil type resins, the silicone resins and other resins which can be dissolved in a solvent and subsequently processed thermally to give tough and durable coatings. It has been found thatinks made from the resins described :above normally possess poor process stability and when fabricated inoprinted resistor form have a negative temperature coefficient of resistance. Further certain of these resins are .what is termed noisy; that is, superfluous noise will result in an electrical circuit employing such resistors mero'us other types of compounds may be condensed with ,epichlorohydrin to form basic resins to beused in acjdroxy biphenyl, p-di-hydroxy benzene, ethylene glycol and higher glycols, bis-p-hydroxy di-benzyl, as well as substituted, particularly chlorinated, derivatives thereof. These resins will vary from viscous liquids to solids with very high melting points.

These epoxy resins are perhaps better known as ethoxyline resins and are characterized in the uncured form by substantially reactive oxide groups at the end of the aliphatic-aromatic chains. Various specific examples of known types of these resins are listed in Letters Patent Nos. 2,324,483 and 2,444,333. Other descriptions of this class of resins may be found in contemporary literature, as for example, British Plastics for November, 1948, at pp. 521, 527 and Electrical Manufacturing" for July, 1949, at pp. 78 through 81 inclusive, 164 and 166.

I times known as param.

Where a solvent is desired, the resins can be dissolved flake, iron carbonyl, ferrites, and other conducting and in butyl carbitol and other ether alcohols; in methyl ethyl which is employed in accordance with the presentjirivention is preferably an amine type and .where possible a ,polyfunctional' amine ;as represented by the formula R( NH2)I where the integer a is from 2 .to 5 is employed; a preferred aminecross linking agent isjihexame thylene diamine. Piperidine is alsoja useful cross-linking agent but isrused only in;those limited cases where short pot life at room temperature can, be tolerated." Much preferred, however, are those cross-linking agents for the final curingand cross-linking of the resin which arelcompletely unreactive at temperaturesless than; 75f 5C and where possible are-solids whichdo not dissolve in the resin mix until the temperature of iat least 100 C. is reached. Thus they'may be presentasfinely divided particles suspended in the.res i,stor ink .i; A preferred crosslinking agent in this catego'ryis dicyandiarnide som e- It is also possible to employ organic alcohols and acids to effect the modification "throughf'the reaction of the available hydrogen from these compounds. Typical ialcohols are, butane-dioland decamethylene glycoljwhile suitable acids include terephthalic acid and 'sebacic' acid.

In cases where the epoxy end group reacts to give a cross-linked resin simply by catalytic effect of the agent notablypiperidine, small amounts of agent give good results. To'improve the stability forprocessing as well as to modify the preferred resin it is preferred to treat the resin with a material e. g. di isocyanate, whichwillreact with the hydroxy group resulting from cross-linking and result in'af final product vpossessing reduced solubility and/or humidity siensitivity. It'is also possible to modify the epoxy resin, prior to lncorporation thereof,into the resistor ink formulation.

One typeof modification involves an esterification'in the epoxy resins may be printed with or without chemical reaction with such resins as the oxidizing alkyd types, the non-oxidizing alkyds, the styrene alkyds and the long alkyds; the phenolic resins, the urea resins, the sulfone resins,.etc. V l

One of the preferred ink compositions of the invention comprises an epoxy resin admixed with a butylated urea formaldehyde resin which may subsequently bereacted by thermalitreatment with or without a catalyst The stability of such prints at room temperature is very good. a 'The following examples of preparation of resistor inks in accordance with the present invention will illustrate various combinations which give outstanding results.

-The general method ofv preparation of resistor links is hereafter set forth: I

1. The epoxy resin is dissolvedwith heating in a high boiling solvent as .butyl Carbitol, butyl Carbitol acetate, butyl Cellosolve or others previously listed, e. g. 85-90 C. with butyl Carbitol acetate.

2. If the cross-linking agent is of the resinous type such as butylated urea formaldehyde or of the type which is inert until heated to accelerated temperatures, it is added to the epoxy resin-solvent mixture, which hasbeen cooled to room temperature. I

3. quantity of a conducting material such-as carbon black and/or graphite, silver flake, copper fiiikQfiflijGk non-ionic wetting agent, e.gJSharples n neiol c"#2d43,

of the-resulting inks follow:

vent, butyl carbitol acetate, was added are:

Per JAN Specs. R 11:

vent, butyl carbit'ol acetate, was added semiconducting compounds is added to the combined resin-solvent mixture in order to obtain the desired resistance value.

.4. In the case of resistance values above 1 x 10 ohms per square, a quantity of an inert filler such as talc, silica, alumina or zirconia maybe added.

5. The various. ingredients are then mixed by hand and/or mechanical stirring until the dry materials are thoroughly wet. The additionf of small quantities'of a may be used.

6. A quantity of a low boiling solventsuch-as acetone or methyl ethyl ketone isuaddedrto obtain the desired viscosity of the ink, and the entire mixture is placed in a porcelain ball mill jar withan appropriate quantity of porcelain, flint or steel balls.

.7. The ink is then milled for a period of time of from I 1 to 10 days, with the temperature'preferably maintained 20 at 25 (3.:5" where a cross linking agent inert until heated to elevated temperature is used. i

8. In some cases 'sufficient mixing may be accomplished by mechanical stirring or the use of a conventional paint mill either with or without the addition of the low boiling solventof step 6. Y

9. The ink is then removed froni theiball'niill jar and I separated from the balls by any pra'cjitical'means.

10. The ink is then vacuum distilled at a low temperature to remove the low boiling solvent, and if necessary quantities of the high boiling solvent are added to'obtain the desired viscosity.

11. The ink is then considered ready for use. I

12. By blending various quantities of inks containing different amounts of conducting materials, inks possessing resistance value between the resistance values of the blendingil ks may bl de. 1

- 13. If the cross-linking agent is other than the resinous or inert types, it is added to the ink'immediately before I the screening operation. r i

Some typical formulations with various characteristics 7 Example I To an ink of the following composition sufiicient solg Percent Epoxy-butylated urea formaldehyde resin solids 79 Lamp black (Wirts calcined) 21 to achieve the desired viscosity of about 35,000"centipoises. The weight ratio of epoxy resin solids to butyl- The Experimentally determined characteristics of this ink Resistance 1 value 7.41 1 0 ohms/ sq.

Voltage coefficient 0.0192'% /volt. Temperature characteristic 1 at 65 c +2.76%: at 105 C +22%.

Noise 1 Less than 1 micro volt/volt. 7

After screening to end of process:

Process change 2 0%. 1 per JAN Specification R-ll. :resistance value changes between initial screened and cured-resistor andfinalprocessede1ement. v

Example II I To an ink of the following composition sufficient sol- Percent Epoxy-butylated urea formaldehyde resin solids a 80 -Lamp black (Wirt?s calcined) a 8.56 lnertfiller (talc).v g 11.44

before screening.

to achieve the desired viscositycf'about' 35,000 centipoises. The 'weightratio of epoxy'resin solids 'to "butylated urea formaldehyde resin solids was 4 to 1. Curing was at 250 C. for ninety minutes.

are:

Resistance value 1 5 .2 10 ohms/ sq. Voltage coeflicient 1 +0.0036%/volt. Temperature characteristic at 65 C +2.53%.

at 105 C '+10.75%.

Noise 1 1.85 micro volts/ volt. Process change 2 +12.2%

1 :per JAN Specification R11. :resistance value changes between initial screened and cured resistor and final processed element.

Examplelll The ink was of the following composition:

i The low boiling solvent was removed by lowitemperature distillation, vacuum distillation at 40'-45 C; at 10 cm. mercury pressure, and the low temperature cross-linking agent was not added until immediately Curing was at 150 C. for ninety minutes.

Experimentally determined characteristics are:

Resistance value 1 3.59 l Voltage coefiicient 1 +0.0521%/volt.

Temperature characteristics 1 at 110 C +5.66%. Noise 1 0.194 micro volt/volt.

1 per J AN Specification R11.

The next three examples were catalyzed just prior to screening the resistor ink upon a non-conducting ceramic base by admixture of the catalyst. Curing was at 250 C. for ninety minutes.

Example IV Ink composition:

Percent Epoxy-butylated urea formaldehyde resin solids 78.4 Lamp black (Wirts calcined) 19.6 Diethylene triamine 2.0

Electrical characteristics:

Process change +12.5%. Noise Less than 1 micro volt/ volt. Voltage coefficient .O29%/volt.

Example V Ink composition:

Percent Epoxy-butylated urea formaldehyde resin solids 78.4 Lamp black (Wirts calcined) 19.6 Piperidine 2.0

Electrical characteristics:

Process change +l5.7%. Noise Less than 1 micro volt/ volt. Voltage coefficient .021%/vo1t.

Example VI Ink composition: 5

Percent Epoxy-butylated urea formaldehyde resin solids--- 78.4 Lamp black (Wirts calcined) 19.6 Dicyandiamide 2.0

' Experimentally determined characteristics of this ink flexible organic bases such as regenerated cellulose, poly-q l 'Elec trical-l characteristics Process change .'14.s%. Noise l;88micro volts/volt.

Voltage coefficient -.018%/ volt.

Epoxy-butylated urea formaldehyde. Phenol-formaldehyde Lamp Black (Wirts calcined). Inert filler (whiting) The aging tests were started immediately after the final processing step, wax impregnation; by maintaining them within the temperature range of 25:5.0" C.

1 Results; i p

Percent Change In Resistance 'Daysoi Aging g i A B A -cured for 90 minutes at 250 C. B cured f0r minutes at 157 C.

According to one ofthe limited embodiments of this invention. the epoxy resin is modified by treatment with organic isocyanate, preferably a polyfunctional isocyanate. Through such treatment with the latter an extremely tough, cross-linked resin can be obtained, possessing unique thermal stability and other desirable properties; suitable compounds are: hexamethylene diisocyanate and tolylene di-isocyanate-2,4.

The above discussion has been directed towards the use of a relatively solid non-porous base for deposition of the resistor. it has been found that a particulanly useful though special form of printed resistor can be obtained by coating a semi-porous and flexible inorganic base with the resistor inks of this invention and by attachment of this flexible base to a fixed base in any desired position and in any desired configuration. Base materials which have been found particularly suitable for this form of the present invention are paper made with fine glass fiber, asbestos fiber paper and glass cloth. The latter has been found particularly satisfactory when thicknesses in the order of 2 to 5 mils are employed. The ink is applied to one side of the flexible cloth and allowed to dry to remove the low boiling solvent, leaving a tacky resinous surface. The cloth thus treated can be then applied to a base or stored by rolling up with polyethylene or polytetrafluoroethylene films. After application of the tacky surface of the coated flexible base against the fixed base and the final removal of the solvent if any be present the curing of the resin is conducted. The exposed and uncoated surface of the flexible base may be treated with resin, preferably the same resin as that employed as the binder in the resistor ink, to protect the resistor from atmospheric conditions.

It is also possible to employ semi-porous or non-porous curing of the resin.

layers after" applic'ationand processingofsthe resistance layer and its'terminals.

Generally. speaking,"the printed resistorsand printed ;circuits employing such resistors, shouldbe providedwith a protective insulating :housing; generallyjthis 'takesrthe 'formof whatiis known as a cover coat. For optimum results it has been found thatthe; cover-moat shou ldncon sist of a resin of the type described hereiiras an ink binder .resin. In suchinstances, of course, a non-conducting filler would be employed. The particular particle size and concentration of the filler, mayx benselected ,so as to modify the thermal expansion characteristics of the,,cover ffco at and toapefniit impregnation prime cover coat with a l wax or other-hydrocarbon moisture barrier after'reinoval of the solvent an dcuring of theres'in. i

In accordance with another limited embodiment of the present:'i1 :\ver ition"theresistor ink binder has' no crosslinking agent in-the ink formulation. After laying down of the resistor and-removal of the solvent, if anybe present, the screened or printed resistor is dipped'in a solution of the cross-linking agent or preferably treated .with its ,vapors (for this purposeammonia or organic amine is particularly suitable) in order to accelerate the In this way, it'is possible to produce resistor inks with indefinite shelf life yet which possess rapid curing characteristics when subsequently processed by the simple and inexpensiveprocedure of treatment with the cross-linking agentf'Preferably this treatment is conducted in the oven usedfor curing the resin; hence no additional equipment is required. In accordance'with another embodiment of the present invention the resistor ink compositionfinclu'dcs a protective. wax or resinQWhich I will bleed-out ofthe resin during thecuring operation and protect the. surface of the latter from the effects of humidity and other adverse conditions. This is accomplished through the reduced solubility of the resin following cure of the said resin to a higher molecular state.

' As many apparently widely different embodiments of this invention maybe made without departing from the sspit t cd. s e h e i is' t be un ers d. a t ,i ,ention'is notlimited to the specificembodiments hereof veaq s efin d n 'c'ap de c aim Whatisclaimedis: I p

1.-'.A- P 1 t4. i Qr,wm ris sa ce m t ba e an adherent layer of resistance material consisting of from about to about of cross-linked epoxy resin, from about-50% to about 5% of finely divided conducting particle's and fromaboiit 25% to about 0% of finely divided non-conducting particle's deposited upo'n said base, and terminal contact areasprovided at spaced portions of said layer.

2. A printed resistor comprising an insulating base and adherent layer of resistance material consisting of about 50% tq abqut 9 5% by weight cross-linked epoxy resin, from about 50% to about5% of finely divided conducting particles and from'about 25 to about 0% of finely divided non-conducting particles deposited upon said base and terminal conducting areas provided at spaced portions of said layer. a v v I T 3. The printed resistor of claim 2 in which the conducting particles are'carbonQ Q4. The printed resistor of claim 2 in which the conducting particles are silver.

References Cited in the file of this patent Y UNITED STATESVPATENTS 1,985,166 Haroldson Dec. 18, 1934 2,120,930v Cooper June 14, 1938 2,258,218 Rochow Oct. 7, 1941 2,437,708 'Plass et a1 Mar. 16, 1948 2,500,449 Bradley Mar. 14, 1950 2,500,600 Bradley Mar. 14, 1950 2,528,360 Greenlee Oct. 3l, 1950 2,692,321 Hicks Oct. 19, 1954 OTHER REFERENCES Circular 

1. A PRINTED RESISTOR COMPRISING A CERAMIC BASE, AN ADHERENT LAYER OF RESISTANCE MATERIAL CONSISTING OF FROM ABOUT 50% TO ABOUT 95% OF CROSS-LINKED EPOXY RESIN, FROM ABOUT 50% TO ABOUT 5% OF FINELY DIVIDED CONDUCTING PARTICLES AND FROM ABOUT 25% TO ABOUT 0% OF FINELY DIVIDED NON-CONDUCTING PARTICLES DEPOSITED UPON SAID BASE, AND TERMINAL CONTACT AREAS PROVIDED AT SPACED PORTIONS OF SAID LAYER. 